Because acoustic ink printers (ALP) avoid the clogging and manufacturing problems of the more conventional drop-on-demand, nozzle-based ink jet printers, they represent a promising direct marking technology. While significant effort has gone into developing acoustic ink printing, see, for example, U.S. Pat. Nos. 4,751,530; 4,751,534; 5,028,937; and 5,041,849, various problems remain to be solved before AIP becomes a viable marking technology.
An acoustic ink printer utilizes acoustic energy to eject droplets from an unbounded surface of a marking fluid onto a recording surface. Typically this involves focusing acoustic energy from an ultrasonic transducer, using either a spherical or a fresnel (reference U.S. Pat. No. 5,041,849) acoustic lens, into a focal area near the unbounded surface. If the acoustic energy is sufficient, an ink droplet having a diameter about the same as the acoustic wavelength is ejected. For a more detailed description of the ejection process reference is made to U.S. Pat. Nos. 4,308,547 and 5,028,937, and the citations therein.
As may be appreciated, acoustic ink printers are sensitive to the spacing between the acoustic energy's focal area and the unbounded surface. Since the acoustic focal plane is generally fixed, it is important that the unbounded surface be properly and accurately positioned. Indeed, since current practice dictates that the acoustic focal area be within about one wavelength of the unbounded surface, typically about 10 micrometers, the position must be very accurately controlled. While various liquid level control structures and techniques have been tried, see, for example, U.S. Pat. No. 5,028,937 (which discusses positioning the unbounded surface with a perforated membrane), and U.S. patent application Ser. No. 07/810,248, filed Dec. 19, 1991 (which discloses an anisotropically etched liquid level control structure having inwardly protruding knife-edged lips), all have their problems.
It would be beneficial to have a liquid level control structure that accurately controls the location of the unbounded surface of a liquid, that is producible at low cost, that allows droplets to be ejected onto a recording medium, and that readily attaches to the other sections of the print head.